1. Field of the Invention
The present invention relates to a method and device for the prevention of disparities or error messages and false responses in IFF (Identification Friend or Foe) type systems.
The invention can be applied especially in IFF identification systems used in the field of military aviation working for example in Mode 4 which is an encrypted interrogation/response mode whose waveform is described in the NATO standardization agreement, STANAG 4193.
Hereinafter in the description, the expression “National Mode” (or “National Secure Mode”) designates an encrypted interrogation/response mode used in national systems of IFF type question-and-answer identification. The waveform of the National Mode has a structure identical to that of Mode 4, but makes use of different enciphering algorithms for encryption.
2. Description of the Prior Art
It may be recalled that, in IFF systems, if the interrogation is done in National Mode, only responders capable of understanding the National Mode will respond. Similarly, if the interrogation is done in Mode 4, then only responders capable of understanding the Mode 4 will respond.
An interrogator chooses to make an interrogation in Mode 4 or in National Mode. A responder does not know in which mode it will be interrogated. Several situations may then arise:                if the responder is equipped only for Mode 4, it will be able to respond only to Mode 4 interrogations, assuming that it has the right keys for decryption,        when the responder is equipped only for a National Mode, it can respond only to interrogations in the National Mode, again if it has the right keys.        
The configuration of current systems may give rise to problems of message recognition and false alarms in operation. For example, when an aircraft equipped with a Mode 4 responder is interrogated by an interrogator using a <<National Mode>>, it often happens that a sound alarm is triggered to warn the pilot of a “disparity”. This disparity is the fact that the interrogation received has the right format but cannot be decoded by the Mode 4 algorithms. The triggering of the sound alarms is described for example in the STANAG 4193.
In certain cases, a “false” response is sent out because the interrogation is decrypted by the Mode 4 algorithms, but its contents are different from those encrypted in National Mode.
The object of the invention relies on a novel approach which gives especially the following results: a responder in National Mode (NM or NSM) does not respond to Mode 4 (M4) interrogations and is not disturbed by a sound alarm. Similarly, a Mode 4 responder does not respond to National Mode interrogations and is not disturbed by a sound alarm when it is interrogated in National Mode.